Minimizing Polymer Degradation

Degradation most often results in the reduction of the molecular weight (MW) of the polymer and the generation of monomers and oligomers. This leads to lower physical properties this, in turn, can lead to out-of-spec conditions for the extruded product. 

The melt temperatures and stresses to which the polymer is exposed in the extruder are strongly influenced by the geometry of the extruder screw. The functions of the extruder screw are 

Currently, in most medical extrusion operations, the mixing of screws is used. This is necessary to make high-quality extruded medical products. Simple conveying screws (without mixing elements) are rarely used anymore because they lead to poor homogeneity in the product, as well as dimensional variation. 

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In search of less expensive, less toxic, and lower viscosity eluants, a few authors have proposed diluting the active ingredient with a common SEC eluant such as toluene, dichloromethane, or chloroform. To lower the operating temperature and minimize polymer degradation, mixtures of m-cresol with chlorobenzene (50 50, v/v, 43°C), dichloromethane (50 50, room temperature), and chloroform have been used, with 0.25 wt% benzoic acid added to prevent adsorption. In the same vein, o-chlorophenol has been diluted with chloroform (25 75) and used at 20°C. The main disadvantage in this latter solvent was a small dnidc for the polymer, which rendered refractive index measurements difficult. In addition, careful purification of the phenol is required to obtain a detection signal. Dichloroacetic acid diluted to 20 vol% with dichloromethane has been proposed as the mobile phase. However, even at this concentration, PA tends to degrade at room temperature. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

T. Masuda, Y. Niwa, A. Tamagawa, S. R. Mukai, K. Hashimoto and Y. Ikeda Degradation of waste poly(ethylene terephthalate) in a steam atmosphere to recover terephthalic acid and to minimize carbonaceous Residue, Polym. Degrad. Stab., 58, 315-320 (1997). 

All aqueous microsphere dispersions must be dried after the capsules are completely hardened. Freeze-drying is a common method of gaining stable powders that can be stored and appHed. It is important to minimize the amount of resting water in the formulation, as not orJy the drug can be hydrolyzed but also the polymer. Polyesters and polyanhydrides both degrade by hydrolysis catalyzed by water consequently, the release mechanism usually depends on drug diffusion as well as on polymer degradation. 

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